1. Field of the Invention
The present invention relates generally to the sheets of metal, such as aluminum, which are used in the construction of honeycomb structures. More particularly, the invention involves the protective coatings or treatments that are applied to the metal sheets prior to formation of the sheets into a honeycomb.
2. Description of Related Art
Honeycomb structures are well known and widely used in many applications where a high strength and lightweight material is required. The combined features of lightweight and strength found in honeycomb structures makes them particularly well suited for use in aircraft and other applications where high strength and low weight are required. Honeycomb structures have been made from a wide variety of materials including metals, such as aluminum and aluminum alloys. Composite materials made from resin impregnated fibers and papers have also been widely used in honeycomb structures.
One common process for fabricating honeycomb structures involves bonding multiple thin sheets of aluminum together along specially oriented node lines. The node lines are offset between different layers in such a way that a honeycomb structure is formed when the layers are expanded. This type of process is commonly referred to as the xe2x80x9cexpansionxe2x80x9d process. The expansion process is not suitable for fabricating honeycomb structures in some instances where certain types of aluminum or aluminum alloys are used. For example, sheets which are relatively thick or are made from certain alloys of aluminum are too stiff and cannot be formed into honeycomb structures using the expansion process.
A fabrication process or method commonly referred to as the xe2x80x9ccorrugationxe2x80x9d process has been used to form high strength honeycomb structures in those situations where the expansion process is not suitable. The corrugation process involves initially shaping sheets of aluminum into a corrugated configuration. The corrugated aluminum sheets are then bonded together along node lines to form the final honeycomb.
In both the expansion and corrugation processes, adhesives are typically used to bond the aluminum sheets together. Such adhesives are commonly referred to as xe2x80x9cnode adhesivesxe2x80x9d. The surfaces of the aluminum sheets are usually coated with a coating or otherwise treated to insure good adhesion between the node adhesive and the aluminum sheet. The strength of the bond between the adhesive and the treated aluminum sheets is generally determined by measuring the peel strength of the adhesive. Peel strength is typically measured using a standard procedure as set forth in ASTMD 1781. The test basically involves gluing two 3-inch (7.6 cm) or two 1-inch (2.54 cm) wide strips of metal sheets together and the measuring of the amount of force required to peel the two strips apart.
Achieving maximum peel strength for a given adhesive is an important goal when manufacturing high strength honeycomb. Failure of the adhesive bond between aluminum sheets can result in complete failure of the entire structure. Accordingly, there is a need to provide surface coatings and/or treatments for aluminum that enhance adhesion of the node adhesive to the surface.
Aluminum is also usually treated to increase resistance to corrosion. Chromate has been a popular corrosion resistant coating for aluminum that has been widely used. However, there are a number of environmental concerns associated with the use of chromate. A number of other corrosion resistant coatings have been developed for aluminum. For example, U.S. Pat. No. 3,687,882 describes coating aluminum with silane-titanante dispersions.
In the art of preparing aluminum honeycomb, there has been and continues to be a need for surface treatments that not only provide good peel strength, but also provide good corrosion protection. Developing a coating system or treatment that provides both of these two desirable features is important in making honeycomb from aluminum which is suitable for use where high strength and light weight is required.
In accordance with the present invention, it was discovered that metals, such as aluminum, can be coated with a two layer system that provides an unexpected combination of good adhesive bonding and corrosion resistance. The two-layer coating includes a first layer located on the surface of the metal which is composed of hydrated alumina and a binder. The second layer of the coating system is located on top of the first layer and includes a polymer sealant. This two-layer coating was found to be especially well suited for use in preparing honeycomb by either the expansion or corrugation process. The present invention not only covers individual sheets of metal having the above-described two-layer coating, but also covers honeycomb structures which are made from numerous sheets which are adhesively bonded together at spaced locations.
The present invention is also directed to methods for making coated metal structures that exhibit increased adhesion with node adhesives and increased corrosion resistance. The method involves applying a coating to the surface of the metal or metal alloy wherein the coating includes a first layer having hydrated alumina dispersed in a binder followed by a second layer located on top of the first layer which is composed of a polymer sealant.
As a feature of the present invention, it was discovered that metals which have the above-described two-layer coating can be used to form adhesively bonded structures that exhibit both high peel strength and corrosion resistance. The peel strength and corrosion resistance of structures having the combined two layer coating was found to be much higher than expected based on the peel strengths and corrosion resistance of structures having only one of the layers.
The above described and many other features and attendant advantages of the present invention will become better understood by reference to the following detailed description when taken in conjunction with the accompanying drawings.